I'm looking for fics where Spike and Giles are related ,any settings ,any universe .They can be father and son or brothers or Giles can be Spike's descendant

There's lots of those. One is my The Song Remains the Same, wherein Giles is Spike's uncle. It's still a WIP (yes, I know! I know!) and can be found here on TSR as well as on Elysian Fields and fanfiction.net.

It Matters to Me by Ashlee

Summary: Buffy Summers has had a crush on Spike Giles since she moved to town. However, Spike never acknowledged Buffy while they were in school together. When Buffy starts working with Spike at the Magic Box, a tentative friendship is formed. The two grow closer until....

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=4658&warning=NC-17

Fire and Ice by Ashlee

Summary: Rewritten – Spike Giles has everything he ever wanted- a beautiful girlfriend, a successful career- living the dream of every man. When his girlfriend discovers something that he's done to destroy their relationship, Spike has to try and win back her trust. But in the harsh light of day, both have to make a decision that will effect the rest of their lives. Is love enough to save a relationship

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=12463

Baby Love by Niamh I think Giles is his uncle in this one.

Summary: Seventeen year old Buffy Summers is a High School Senior with a boyfriend and life is great, until she realizes one morning that, well, life changes fairly quickly, especially when you find out something you've been denying is real. Nominated in the 9th Round of the Love's Last Glimpse Awards for: Best WIP, Best Angst and Best Fantasy

http://spikeluver.com/SpuffyRealm/viewstory.php?sid=10127

Protect Me From What I Want by rae (Giles is Spike's dad, Buffy/Spike step siblings) there is a sequel to this Protège Moi.

Been Here Too Few Years by Addie Logan (Giles Spike's dad again, Buffy/Spike also step siblings again)

Silently Broken by suzee (Giles is Spike's brother and Buffy's stepfather)

Thank you all for your great efforts

You're welcome.

There is also Beautiful Mistakes by Spikes_Slayer08. It's here on this site. I put the summary below. I read it not to long ago and I want to say they are father/son in this one.

Beautiful Mistakes

Buffy and Spike have been best friends for as long as they both can remember. After one passion filled night together Buffy and Spike find themselves in uncharted waters. Cheating partners, a bottle of Jack Daniels, tight shirts, jeans and entangled limbs. Was the night all one big mistake? Will Spike realise Buffy's true feelings before he looses her forever? or will it all just be to little to late?